Positive electrode active material and nonaqueous electrolyte secondary battery using the same

ABSTRACT

Provided is a positive electrode active material that can increase a volume capacity density of a positive electrode of a nonaqueous battery secondary battery and can provide the nonaqueous electrolyte secondary battery with high cycle characteristics. A positive electrode active material disclosed here includes monoparticulate first lithium composite oxide particles and secondary particulate second lithium composite oxide particles. An average particle size of the second lithium composite oxide particles is larger than an average particle size of the first lithium composite oxide particles. The second lithium composite oxide particles have a porosity of 0.9% to 4.0%.

TECHNICAL FIELD

The present disclosure relates to a positive electrode active material.The present disclosure also relates to a nonaqueous electrolytesecondary battery using the positive electrode active material. Thisapplication claims the benefit of priority to Japanese PatentApplication No. 2021-206372 filed on Dec. 20, 2021. The entire contentsof this application are hereby incorporated herein by reference.

BACKGROUND

Recently, nonaqueous electrolyte secondary batteries such as lithium ionsecondary batteries are suitably used for, for example, portable powersupplies for devices such as personal computers and portable terminals,and vehicle driving power supplies for vehicles such as battery electricvehicles (BEV), hybrid electric vehicles (HEV), and plug-in hybridelectric vehicles (PHEV).

With widespread use of nonaqueous electrolyte secondary batteries, thesebatteries are required of further enhanced performance. A positiveelectrode of a nonaqueous electrolyte secondary battery generallyemploys a lithium composite oxide as a positive electrode activematerial. To enhance performance of a nonaqueous electrolyte secondarybattery, a technique of mixing two types of lithium composite oxideshaving different particle properties is known (see, for example, PatentDocuments 1 and 2). In Patent Documents 1 and 2, for example, amonoparticulate lithium composite oxide and an agglomerated (i.e.,secondary) particulate lithium composite oxide are used.

CITATION LIST Patent Documents

Patent Document 1: WO2021/065162

Patent Document 2: JP 2019-160571A

SUMMARY

Inventors of the present disclosure have intensively studied the use ofmonoparticulate lithium composite oxide and secondary particulatelithium composite oxide in combination, and have found that a positiveelectrode of a nonaqueous electrolyte secondary battery in aconventional technique has an insufficient volume capacity density. Onthe other hand, the nonaqueous electrolyte secondary battery is requiredto have high cycle characteristics.

It is therefore an object of the present disclosure to provide apositive electrode active material that can increase volume capacitydensity of a positive electrode of a nonaqueous electrolyte secondarybattery and can provide the nonaqueous electrolyte secondary batterywith high cycle characteristics.

A positive electrode active material disclosed here includes:monoparticulate first lithium composite oxide particles; and secondaryparticulate second lithium composite oxide particles. An averageparticle size of the second lithium composite oxide particles is largerthan an average particle size of the first lithium composite oxideparticles. The second lithium composite oxide particles have a porosityof 0.9% to 4.0%.

With this configuration, it is possible to provide a positive electrodeactive material that can increase volume capacity density of a positiveelectrode of a nonaqueous battery secondary battery and can provide thenonaqueous electrolyte secondary battery with high cyclecharacteristics.

In a desired aspect of the positive electrode active material disclosedhere, an average particle size (D50) of the second lithium compositeoxide particles is 12 μm to 20 μm. This configuration further increasesfilling property of the positive electrode active material including thefirst lithium composite oxide particles and the second lithium compositeoxide particles.

In a desired aspect of the positive electrode active material disclosedhere, the average particle size (D50) of the first lithium compositeoxide particles is 2 μm to 6 μm. This configuration can further increasefilling property of the positive electrode active material including thefirst lithium composite oxide particles and the second lithium compositeoxide particles, and also can provide a nonaqueous electrolyte secondarybattery with high output characteristics.

In a desired aspect of the positive electrode active material disclosedhere, each of the first lithium composite oxide particles and the secondlithium composite oxide particles is particles of a lithium compositeoxide containing Ni and having a layered structure.

In a desired aspect of the positive electrode active material disclosedhere, each of the first lithium composite oxide particles and the secondlithium composite oxide particles is particles of a lithium nickelcobalt manganese composite oxide. This configuration can provide thenonaqueous electrolyte secondary battery with more excellent batterycharacteristics such as small initial resistance.

In a more desired aspect of the positive electrode active materialdisclosed here, a content of nickel in all metal elements except forlithium in the lithium nickel cobalt manganese composite oxide is 50 mol% or more. This configuration can provide the nonaqueous electrolytesecondary battery with a high volume energy density.

In another aspect, a nonaqueous electrolyte secondary battery disclosedhere includes a positive electrode, a negative electrode, and anonaqueous electrolyte. The positive electrode includes theabove-described positive electrode active material. This configurationcan provide the nonaqueous electrolyte secondary battery having a highvolume capacity density of the positive electrode and high cyclecharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of monoparticles.

FIG. 2 is a schematic view of secondary particles.

FIG. 3 is a cross-sectional view schematically illustrating an internalstructure of a lithium ion secondary battery according to one embodimentof the present disclosure.

FIG. 4 is a schematic disassembled view illustrating a structure of awound electrode body of a lithium ion secondary battery according to oneembodiment of the present disclosure.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described hereinafterwith reference to the drawings. Matters not specifically mentionedherein but required for carrying out the present disclosure can beunderstood as matters of design variation of a person skilled in the artbased on related art in the field. The present disclosure can be carriedout on the basis of the contents disclosed in the description and commongeneral knowledge in the field. In the drawings, members and partshaving the same functions are denoted by the same reference charactersfor description. Dimensional relationships (e.g., length, width, andthickness) in the drawings do not reflect actual dimensionalrelationships. A numerical range expressed as “A to B” herein includes Aand B.

A “secondary battery” herein refers to a power storage device capable ofbeing repeatedly charged and discharged, and includes a so-calledstorage battery and a power storage element such as an electric doublelayer capacitor. A “lithium ion secondary battery” herein refers to asecondary battery that uses lithium ions as charge carriers and performscharge and discharge by movement of charges accompanying lithium ionsbetween positive and negative electrodes.

A positive electrode active material according to this embodimentincludes monoparticulate first lithium composite oxide particles andsecondary particulate second lithium composite oxide particles.

The crystal structure of lithium composite oxide constituting each ofthe first lithium composite oxide particles and the second lithiumcomposite oxide particles is not specifically limited, and may be, forexample, a layered structure or a spinel structure.

The lithium composite oxide is desirably a lithium transition metalcomposite oxide including at least one of Ni, Co, or Mn as a transitionmetal element, and specific examples of the lithium composite oxideinclude a lithium nickel composite oxide, a lithium cobalt compositeoxide, a lithium manganese composite oxide, a lithium nickel manganesecomposite oxide, a lithium nickel cobalt manganese composite oxide, alithium nickel cobalt aluminium composite oxide, and a lithium ironnickel manganese composite oxide.

The “lithium nickel cobalt manganese composite oxide” herein includesnot only oxides including Li, Ni, Co, Mn, and O as constituent elements,but also an oxide further including one or more additive elementsbesides them. Examples of the additive elements include transition metalelements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y,Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additive element may be ametalloid element such as B, C, Si, or P, and a nonmetal element such asS, F, Cl, Br, or I. This also applies, in the same manner, to, forexample, lithium nickel composite oxide, lithium cobalt composite oxide,lithium manganese composite oxide, lithium nickel manganese compositeoxide, lithium nickel cobalt aluminium composite oxide, and lithium ironnickel manganese composite oxide.

The lithium composite oxide is more desirably a lithium composite oxidecontaining nickel (Ni) and having a layered crystal structure (i.e.,Ni-containing lithium composite oxide having a layered structure). Thefact that a lithium composite oxide has a layered crystal structure canbe confirmed by a known method (e.g., X-ray diffraction).

Among Ni-containing lithium composite oxides having layered structures,a lithium nickel cobalt manganese composite oxide is desirable becauseof excellent properties such as low initial resistance. From theviewpoint of a high volume energy density of the nonaqueous electrolytesecondary battery, the content of nickel in the all metal elementsexcept for lithium in the lithium nickel cobalt manganese compositeoxide is desirably 50 mol % or more, and more desirably 55 mol % ormore.

Specifically, the lithium nickel cobalt manganese composite oxidedesirably has the composition expressed by Formula (I):

Li_(1+x)Ni_(y)Co_(z)Mn_((1−y−z))M_(α)O_(2−β)Q_(β)  (I)

In Formula (I), x, y, z, α, and β respectively satisfy −0.3≤x≤0.3,0.1<y<0.9, 0<z<0.5, 0≤α≤0.1, and 0≤β≤0.5. M is at least one elementselected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn,Sn, B, and Al. Q is at least one element selected from the groupconsisting of F, Cl, and Br.

From the viewpoint of a high energy density of the nonaqueouselectrolyte secondary battery, y and z desirably satisfy 0.50≤y≤0.88 and0.10≤z≤0.45, and more desirably satisfy 0.55≤y≤0.85 and 0.10≤z≤0.40.

The first lithium composite oxide particles and the second lithiumcomposite oxide particles may have the same composition or differentcompositions. Because of excellent properties such as small initialresistance, both of the first lithium composite oxide particles and thesecond lithium composite oxide particles are desirably particles of alithium nickel cobalt manganese composite oxide.

The first lithium composite oxide particles are in a form ofmonoparticles (i.e., monoparticulate particles). The “monoparticles”herein refer to particles generated with growth of single crystalnuclei, and thus, refer to particles of single crystal including nograin boundary. The state where particles are single crystal can beconfirmed by analysis of an electron diffraction image obtained by atransmission electron microscope (TEM), for example.

Monoparticles have the property of having difficulty in agglomeration,and constitute lithium composite oxide particles by themselves, but canbe agglomerated to form lithium composite oxide particles in some cases.It should be noted that in the case where monoparticles are agglomeratedto form lithium composite oxide particles, the number of agglomeratedmonoparticles is two or more and ten or less. Thus, one lithiumcomposite oxide particle is constituted by one or more and ten or lessmonoparticles, can be constituted by one or more and five or lessmonoparticles, can be constituted by one or more and three or lessmonoparticles, or can be constituted by one monoparticle. The number ofmonoparticles in one lithium composite oxide particle can be confirmedby observation with a scanning electron microscope (SEM) at amagnification of 10,000 to 30,000.

Thus, monoparticles can be schematically illustrated in FIG. 1 . FIG. 1shows isolated particles, and particles as agglomerated particles. Onthe other hand, secondary particles can be schematically shown asillustrated in FIG. 2 . In FIG. 2 , a large number of particles areagglomerated to form one particle. Secondary particles are typicallyconstituted by at least 11 or more primary particles. FIGS. 1 and 2 areexample, and first lithium composite oxide particles and second lithiumcomposite oxide particles used in this embodiment are not limited to theillustrated particles.

In the manner described above, monoparticles are different frompolycrystalline particles constituted by a plurality of crystal grainsand secondary particles formed by a large number of agglomerated fineparticles (primary particles). A monoparticulate positive electrodeactive material can be produced with a known method for obtaining singlecrystalline particles.

The second lithium composite oxide particles are secondary particles inwhich primary particles are agglomerated. Typically, secondary particles(i.e., second lithium composite oxide particles) can have inner voidsderived from gaps between agglomerated primary particles. With respectto these inner voids, the second lithium composite oxide particles havea porosity of 0.9% or more and 4.0% or less in this embodiment.

In this embodiment, an average particle size (D50) of the second lithiumcomposite oxide particles is larger than an average particle size (D50)of the first lithium composite oxide particles. Thus, in thisembodiment, monoparticles having small particle sizes and secondaryparticles having large particle sizes are used as positive electrodeactive materials.

This configuration can increase a volume capacity density of a positiveelectrode of a nonaqueous electrolyte secondary battery and can providethe nonaqueous electrolyte secondary battery with high cyclecharacteristics (especially resistance to capacity degradation inrepetitive charging and discharging).

Specifically, the use of monoparticles having small particle sizes andsecondary particles having large particle sizes in combination canenhance filling property of particles constituting the positiveelectrode active material. Since the positive electrode active materialincludes monoparticles, cycle characteristics of the nonaqueouselectrolyte secondary battery can be enhanced.

Here, the volume capacity density increases as the density of thepositive electrode active material increases. A nonaqueous electrolytesecondary battery is required to have high volume energy density, andthe volume energy density increases as the volume capacity densityincreases. To meet the demand for high volume energy density, particleshaving a very small void amount (specifically particles generally havinga porosity of 0.5% or less) are generally used as particles of apositive electrode active material in order to increase the density(bulk density) of the positive electrode active material.

A study of the inventors of the present disclosure has, however,revealed that when the second lithium composite oxide particles have asmall porosity, capacity significantly degrades in repetitive chargingand discharging of a nonaqueous electrolyte secondary battery. This isbecause stress generated in the repetitive charging and discharging ofthe nonaqueous electrolyte secondary battery cannot be reduced so thatsecondary particles are thereby cracked to generate isolated primaryparticles. In view of this, in this embodiment, the second lithiumcomposite oxide particles have a porosity higher than usual.Specifically, in this embodiment, the second lithium composite oxideparticles have a porosity of 0.9% or more, and the presence of anappropriate amount of voids can reduce the stress described above.

On the other hand, if the second lithium composite oxide particles havean excessively large porosity, the volume capacity density of thepositive electrode decreases. In addition, in a case where the positiveelectrode active material layer of the nonaqueous electrolyte secondarybattery has a high density, press treatment for increasing the densitycauses cracks in the second lithium composite oxide particles, and thus,the amount of gas generation increases due to the portion wheresecondary particles crack. In view of this, in this embodiment, thesecond lithium composite oxide particles have a porosity of 4.0% orless.

From the viewpoint of especially high cycle characteristics, theporosity of the second lithium composite oxide particles is desirably1.0% or more, and more desirably 1.1% or more. On the other hand, fromthe viewpoint of especially high volume capacity density, the porosityof the second lithium composite oxide particles is desirably 3.6% orless, more desirably 3.4% or less, and even more desirably 3.2% or less.

It should be noted that the porosity of the second lithium compositeoxide particles can be determined in the following manner. Across-sectional observation sample of lithium composite oxide particlesis prepared by, for example, cross-section polisher process, and an SEMimage thereof is obtained with a scanning electron microscope (SEM).From the SEM image, an area of the entire secondary particles and atotal area of all the voids in the secondary particles are obtained.From (total area of all voids/area of entire secondary particles)×100, aporosity (%) is calculated.

It should be noted that the porosity of the second lithium compositeoxide particles can be adjusted by changing synthesis conditions insynthesizing a hydroxide as a precursor of the second lithium compositeoxide particles by a crystallization technique. Specifically, in acrystallization technique, a raw material aqueous solution containing ametal element except for lithium and a pH adjusting solution are addedto a reaction solution, thereby synthesizing a hydroxide. By changingthe pH of the reaction solution and a steering speed at this time, theporosity of the hydroxide can be adjusted. This hydroxide and a compound(e.g., lithium carbonate) serving as a lithium source are mixed andfired, thereby obtaining second lithium composite oxide particles havingan adjusted porosity.

The average particle size (D50) of the first lithium composite oxideparticles is not specifically limited. From the viewpoints of especiallyhigh filling property of the positive electrode active material layerand high output characteristics of the nonaqueous electrolyte secondarybattery, the average particle size (D50) of the first lithium compositeoxide particles is desirably 2 μm to 6 μm, and more desirably 3 μm to 5μm.

The average particle size (D50) of the second lithium composite oxideparticles is not specifically limited. The average particle size (D50)of the second lithium composite oxide particles is desirably 12 μm to 20μm, more desirably 14 μm to 20 μm, and even more desirably 14.5 μm to 18μm. In the case where the average particle size (D50) of the secondlithium composite oxide particles is within the desired range describedabove, filling property of the positive electrode active material layerincluding the first lithium composite oxide particles and the secondlithium composite oxide particles is increased so that the volume energydensity of the nonaqueous electrolyte secondary battery can be therebyespecially increased.

It should be noted that the “average particle size (D50) of lithiumcomposite oxide particles” herein refers to a median particle size(D50), and means a particle size corresponding to a cumulative frequencyof 50 vol % from the small-size particle side in volume-based particlesize distribution based on a laser diffraction and scattering method.Thus, the average particle size (D50) can be obtained by using, forexample, a particle size distribution analyzer of a laser diffractionand scattering type.

An average primary particle size of the second lithium composite oxideparticles is not specifically limited, and is, for example, 0.05 μm to2.5 μm. To enhance gas generation suppressing performance during storageof the nonaqueous electrolyte secondary battery, the average primaryparticle size is desirably 1.2 μm or more, more desirably 1.5 μm ormore, and even more desirably 1.7 μm or more. On the other hand, fromthe viewpoint of higher cycle characteristics of the nonaqueouselectrolyte secondary battery, the average primary particle size isdesirably 2.2 μm or less, and more desirably 2.1 μm or less.

It should be noted that the “average primary particle size of the secondlithium composite oxide particles” denotes an average longer diameter of50 or more primary particles which are perceived in an electronmicroscopic cross-sectional image of the second lithium composite oxideparticles, and are arbitrarily selected. Thus, the average primaryparticle size can be obtained by, for example, preparing across-sectional observation sample of lithium composite oxide particlesby a cross-section polisher process, acquiring an SEM image of thesample with a scanning electron microscope (SEM), and determining longerdiameters of 50 or more arbitrarily selected primary particles withimage analysis type particle size distribution measurement software(e.g., “Mac-View”) to calculate an average value of these longerdiameters.

It should be noted that the average primary particle size of the secondlithium composite oxide particles can be controlled in the followingmanner. First, according to a known method, a hydroxide as a precursorof second lithium composite oxide particles is prepared. The hydroxidegenerally includes metal elements except for lithium in metal elementsincluded in the second lithium composite oxide particles. The hydroxideand a compound (e.g., lithium carbonate) serving as a lithium source aremixed and fired. By adjusting the temperature and time of this firing,the average primary particle size of the second lithium composite oxideparticles can be controlled. The firing temperature is desirably 700° C.to 1000° C. The firing time is desirably 3 hours to 7 hours.

A BET specific surface area of the first lithium composite oxideparticles is not specifically limited. To provide the nonaqueouselectrolyte secondary battery with excellent output characteristics, theBET specific surface area of the first lithium composite oxide particlesis desirably 0.50 m²/g to 0.85 m²/g, and more desirably 0.55 m²/g to0.80 m²/g.

The BET specific surface area of the second lithium composite oxideparticles is not specifically limited. To provide the nonaqueouselectrolyte secondary battery with excellent output characteristics, theBET specific surface area of the second lithium composite oxideparticles is desirably 0.10 m²/g to 0.30 m²/g, and more desirably 0.13m²/g to 0.27 m²/g.

It should be noted that the BET specific surface areas of the first andsecond lithium composite oxide particles can be measured by a nitrogenadsorption method with a commercially available surface area analyzer(e.g., “Macsorb Model-1208,” manufactured by Mountech Co., Ltd.).

From the viewpoint of a high volume energy density, the content ofnickel in the all metal elements except for lithium in the lithiumnickel cobalt manganese composite oxide is desirably 55 mol % or more inthe first lithium composite oxide particles and 50 mol % or more in thesecond lithium composite oxide particles, and is more desirably 60 mol %or more in the first lithium composite oxide particles and 55 mol % ormore in the second lithium composite oxide particles.

The contents of the first lithium composite oxide particles and thesecond lithium composite oxide particles are not specifically limited.The mass ratio thereof (i.e., first lithium composite oxideparticles:second lithium composite oxide particles) is, for example,10:90 to 90:10, desirably 20:80 to 80:20, more desirably 30:70 to 70:30,and much more desirably 30:70 to 60:40.

The positive electrode active material consists only of the firstlithium composite oxide particles and the second lithium composite oxideparticles. The positive electrode active material may further includeanother particles functioning as a positive electrode active material inaddition to the first lithium composite oxide particles and the secondlithium composite oxide particles.

The positive electrode active material according to this embodiment canincrease the volume capacity density of the positive electrode of thenonaqueous electrolyte secondary battery and can provide the nonaqueouselectrolyte secondary battery with high cycle characteristics(especially, resistance to capacity degradation after repetitivecharging and discharging). The positive electrode active materialaccording to this embodiment is typically a positive electrode activematerial for a nonaqueous electrolyte secondary battery, and isdesirably a positive electrode active material for a nonaqueous lithiumion secondary battery. The positive electrode active material accordingto this embodiment can also be used as a positive electrode activematerial for an all-solid-state secondary battery.

In another aspect, the nonaqueous electrolyte secondary batteryaccording to this embodiment includes a positive electrode, a negativeelectrode, and a nonaqueous electrolyte. The positive electrode includesthe positive electrode active material described above. In thenonaqueous electrolyte secondary battery according to this embodiment,the positive electrode typically includes a positive electrode currentcollector and a positive electrode active material layer supported onthe positive electrode current collector, and the positive electrodeactive material layer includes the positive electrode active materialdescribed above.

The nonaqueous electrolyte secondary battery according to thisembodiment will now be described using an example of a flat squarelithium ion secondary battery including a flat wound electrode body anda flat battery case. The nonaqueous electrolyte secondary batteryaccording to this embodiment, however, is not limited the followingexamples.

A lithium ion secondary battery 100 illustrated in FIG. 3 is a sealedbattery in which a flat wound electrode body 20 and a nonaqueouselectrolyte (not shown) are housed in a flat square battery case (i.e.,outer container) 30. The battery case 30 includes a positive electrodeterminal 42 and a negative electrode terminal 44 for externalconnection, and a thin safety valve 36 configured such that when theinternal pressure of the battery case 30 increases to a predeterminedlevel or more, the safety valve 36 releases the internal pressure. Thepositive and negative electrode terminals 42 and 44 are electricallyconnected to positive and negative electrode current collector plates 42a and 44 a, respectively. A material for the battery case 30 is, forexample, a metal material that is lightweight and has high thermalconductivity, such as aluminium. A current interrupt device (CID) may bedisposed between the positive electrode terminal 42 and the positiveelectrode current collector plate 42 a or between the negative electrodeterminal 44 and the negative electrode current collector plate 44 a.

As illustrated in FIGS. 3 and 4 , in the wound electrode body 20, apositive electrode sheet 50 and a negative electrode sheet 60 arestacked with two long separator sheets 70 interposed therebetween andwound in the longitudinal direction. In the positive electrode sheet 50,a positive electrode active material layer 54 is formed on one or each(each in this example) surface of a long positive electrode currentcollector 52 along the longitudinal direction. In the negative electrodesheet 60, a negative electrode active material layer 64 is formed on oneor each (each in this example) surface of a long negative electrodecurrent collector 62 along the longitudinal direction.

The positive electrode active material layer non-formed portion 52 a(i.e., a portion where no positive electrode active material layer 54 isformed and the positive electrode current collector 52 is exposed) and anegative electrode active material layer non-formed portion 62 a (i.e.,a portion where no negative electrode active material layer 64 is formedand the negative electrode current collector 62 is exposed) extend offoutward from both ends of the wound electrode body 20 in the windingaxis direction (i.e., sheet width direction orthogonal to thelongitudinal direction). The positive electrode active material layernon-formed portion 52 a and the negative electrode active material layernon-formed portion 62 a function as current collection portions. Thepositive electrode current collector plate 42 a and the negativeelectrode current collector plate 44 a are respectively joined to thepositive electrode active material layer non-formed portion 52 a and thenegative electrode active material layer non-formed portion 62 a. Theshapes of the positive electrode active material layer non-formedportion 52 a and the negative electrode active material layer non-formedportion 62 a are not limited to the illustrated example. The positiveelectrode active material layer non-formed portion 52 a and the negativeelectrode active material layer non-formed portion 62 a may be formed ascurrent collection tabs processed into predetermined shapes.

The positive electrode current collector 52 may be a known positiveelectrode current collector for use in a lithium ion secondary battery,and examples of the positive electrode current collector 52 includesheets or foil of highly conductive metals (e.g., aluminium, nickel,titanium, and stainless steel). The positive electrode current collector52 is desirably aluminium foil.

Dimensions of the positive electrode current collector 52 are notspecifically limited, and may be appropriately determined depending onbattery design. In the case of using aluminium foil as the positiveelectrode current collector 52, the thickness thereof is notspecifically limited, and is, for example, 5 μm or more and 35 μm orless, and desirably 7 μm or more and 20 μm or less.

The positive electrode active material layer 54 includes a positiveelectrode active material. As the positive electrode active material,the positive electrode active material according to this embodimentdescribed above is used. The positive electrode active material layer 54may include other positive electrode active materials in addition to thepositive electrode active material according to this embodiment, to theextent that does not inhibit advantages of the present disclosure.

The positive electrode active material layer 54 may include componentsother than the positive electrode active material, such as trilithiumphosphate, a conductive agent, and a binder. Desired examples of theconductive material include carbon black such as acetylene black (AB)and other carbon materials (e.g., graphite). Examples of the binderinclude polyvinylidene fluoride (PVDF).

The content of the positive electrode active material in the positiveelectrode active material layer 54 (i.e., content of the positiveelectrode active material in the total mass of the positive electrodeactive material layer 54) is not specifically limited, and is desirably70 mass % or more, more desirably 80 mass % or more and 99 mass % orless, and much more desirably 85 mass % or more and 98 mass % or less.The content of the conductive material in the positive electrode activematerial layer 54 is not specifically limited, and is desirably 0.5 mass% or more and 15 mass % or less, and more desirably 1 mass % or more and10 mass % or less. The content of the binder in the positive electrodeactive material layer 54 is not specifically limited, and is desirably0.5 mass % or more and 15 mass % or less, and more desirably 0.8 mass %or more and 10 mass % or less.

The thickness of the positive electrode active material layer 54 is notspecifically limited, and is, for example, 10 μm or more and 300 μm orless, and desirably 20 μm or more and 200 μm or less.

The density of the positive electrode active material layer 54 is notspecifically limited, and from the viewpoint of especially high volumeenergy density, is desirably 3.00 g/cm³ to 4.00 g/cm³, more desirably3.20 g/cm³ to 4.00 g/cm³, even more desirably 3.40 g/cm³ to 4.00 g/cm³,and especially desirably 3.50 g/cm³ to 4.00 g/cm³. In this embodiment,since the second lithium composite oxide particles have a porosity of4.0% or less, even when the positive electrode active material layer 54has a high density of 3.00 g/cm³ or more, cracks are less likely tooccur in the second lithium composite oxide particles, and high volumeenergy density can be easily obtained.

In a positive electrode active material layer non-formed portion 52 a ofa positive electrode sheet 50, an insulating protective layer (notshown) including insulating particles may be provided at a locationadjacent to the positive electrode active material layer 54. Thisprotective layer can prevent short circuit between the positiveelectrode active material layer non-formed portion 52 a and the negativeelectrode active material layer 64.

As the negative electrode current collector 62, a known negativeelectrode current collector for use in a lithium ion secondary batterymay be used, and examples of the negative electrode current collectorinclude sheets or foil of highly conductive metals (e.g., copper,nickel, titanium, and stainless steel). The negative electrode currentcollector 62 is desirably copper foil.

Dimensions of the negative electrode current collector 62 are notspecifically limited, and may be appropriately determined depending onbattery design. In the case of using copper foil as the negativeelectrode current collector 62, the thickness thereof is notspecifically limited, and is, for example, 5 μm or more and 35 μm orless, and desirably 7 μm or more and 20 μm or less.

The negative electrode active material layer 64 includes a negativeelectrode active material. Examples of the negative electrode activematerial include carbon materials such as graphite, hard carbon, andsoft carbon. Graphite may be natural graphite or artificial graphite,and may be amorphous carbon-coated graphite in which graphite is coatedwith an amorphous carbon material.

The average particle size (median particle size: D50) of the negativeelectrode active material is not specifically limited, and is, forexample, 0.1 μm or more and 50 μm or less, desirably 1 μm or more and 25μm or less, and more desirably 5 μm or more and 20 μm or less. It shouldbe noted that the average particle size (D50) of the negative electrodeactive material can be determined by, for example, a laser diffractionand scattering method.

The negative electrode active material layer 64 can include componentsother than the active material, such as a binder or a thickener.Examples of the binder include styrene-butadiene rubber (SBR) andpolyvinylidene fluoride (PVDF). Examples of the thickener includecarboxymethyl cellulose (CMC).

The content of the negative electrode active material in the negativeelectrode active material layer 64 is desirably 90 mass % or more, andmore desirably 95 mass % or more and 99 mass % or less. The content ofthe binder in the negative electrode active material layer 64 isdesirably 0.1 mass % or more and 8 mass % or less, and more desirably0.5 mass % or more and 3 mass % or less. The content of the thickener inthe negative electrode active material layer 64 is desirably 0.3 mass %or more and 3 mass % or less, and more desirably 0.5 mass % or more and2 mass % or less.

The thickness of the negative electrode active material layer 64 is notspecifically limited, and is, for example, 10 μm or more and 300 μm orless, and desirably 20 μm or more and 200 μm or less.

Examples of the separator 70 include a porous sheet (film) of a resinsuch as polyethylene (PE), polypropylene (PP), polyester, cellulose, andpolyamide. The porous sheet may have a single-layer structure or alaminated structure of two or more layers (e.g., three-layer structurein which PP layers are stacked on both surfaces of a PE layer). Aheat-resistance layer (HRL) may be provided on a surface of theseparator 70.

A nonaqueous electrolyte 80 typically includes a nonaqueous solvent anda supporting electrolyte (electrolyte salt). As the nonaqueous solvent,various organic solvents such as carbonates, ethers, esters, nitriles,sulfones, and lactones for use in an electrolyte of a typical lithiumion secondary battery can be used without any particular limitation.Specific examples of such a nonaqueous solvent include ethylenecarbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC),dimethyl carbonate (DMC), ethyl methyl carbonate (EMC),monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC),monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethylcarbonate (TFDMC). Such nonaqueous solvents may be used alone or two ormore of them may be used in combination.

Desired examples of the supporting electrolyte include lithium salts(desirably LiPF₆) such as LiPF₆, LiBF₄, and lithiumbis(fluorosulfonyl)imide (LiFSI). The concentration of the supportingelectrolyte is desirably 0.7 mol/L or more and 1.3 mol/L or less.

The nonaqueous electrolyte 80 may include components not describedabove, for example, various additives exemplified by: a film formingagent such as vinylene carbonate (VC) and an oxalato complex; a gasgenerating agent such as biphenyl (BP) or cyclohexylbenzene (CHB); and athickener, to the extent that the effects of the present disclosure arenot significantly impaired.

The thus-configured lithium ion secondary battery 100 has high volumecapacity density of the positive electrode and high cyclecharacteristics (especially, resistance to capacity degradation inrepetitive charging and discharging). The lithium ion secondary battery100 also has high cycle characteristics (especially resistance tocapacity degradation in repetitive charging and discharging). Thelithium ion secondary battery 100 is applicable to various applications.Specific examples of application of the lithium ion secondary battery100 include: portable power supplies for personal computers, portableelectronic devices, portable terminals, and other devices; vehicledriving power supplies for vehicles such as electric vehicles (BEVs),hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles(PHEVs); and storage batteries for small power storage devices, andamong these devices, vehicle driving power supplies are especiallydesirable. The lithium ion secondary battery 100 can be used in abattery pack in which a plurality of batteries are typically connectedin series and/or in parallel.

The foregoing description is directed to the square lithium ionsecondary battery 100 including the flat wound electrode body 20 as anexample. Alternatively, the nonaqueous electrolyte secondary batterydisclosed here can also be configured as a lithium ion secondary batteryincluding a stacked-type electrode body (i.e., electrode body in which aplurality of positive electrodes and a plurality of negative electrodesare alternately stacked). The stacked-type electrode body may include aplurality of separators each interposed between one positive electrodeand one negative electrode or may include one separator in such a mannerthat positive electrodes and negative electrodes are alternately stackedwith one separator being repeatedly folded.

The nonaqueous electrolyte secondary battery disclosed here may beconfigured as a coin type lithium ion secondary battery, a button typelithium ion secondary battery, a cylindrical lithium ion secondarybattery, or a laminated case type lithium ion secondary battery. Thenonaqueous electrolyte secondary battery disclosed here may beconfigured as a nonaqueous electrolyte secondary battery other than alithium ion secondary battery, by a known method.

On the other hand, the positive electrode active material according tothis embodiment can be used for constituting an all-solid-statesecondary battery (especially all-solid-state lithium ion secondarybattery) according to a known method by using a solid electrolyteinstead of the nonaqueous electrolyte 80.

Examples of the present disclosure will now be described, but are notintended to limit the present disclosure to these examples.

Example 1

As first lithium composite oxide particles, monoparticulateLiNi_(0.6)Co_(0.2)Mn_(0.2)O₂ having an average particle size (D50) of3.9 μm and a BET specific surface area of 0.60 m²/g was prepared. Assecond lithium composite oxide particles, secondary particulateLiNi_(0.55)Co_(0.20)Mn_(0.25)O₂ having an average primary particle sizeof 1.8 μm, an average particle size (D50) of 16.5 μm, a BET specificsurface area of 0.20 m²/g, and a porosity of 1.1% was prepared. Itshould be noted that the average primary particle size, the averageparticle size (D50), the BET specific surface area, and the porositywere measured by a method described later.

The first lithium composite oxide particles and the second lithiumcomposite oxide particles were mixed in a mass ratio of 50:50, therebypreparing a positive electrode active material. This positive electrodeactive material, carbon black as a conductive agent, and polyvinylidenefluoride (PVDF) as a binder were mixed in a mass ratio of positiveelectrode active material:AB:PVDF=97.5:1.5:1.0. The resulting mixturewas supplemented with an appropriate amount of N-methyl-2-pyrrolidone(NMP), thereby preparing positive electrode active material layerslurry.

The positive electrode active material layer slurry was applied to bothsurfaces of a positive electrode current collector of aluminium foil,and dried, thereby forming a positive electrode active material layer.The positive electrode active material layer was roll-pressed by rollingrollers to have a density of 3.50 g/cm³, and then cut into apredetermined size, thereby obtaining a positive electrode sheet.

Graphite (C) as a negative electrode active material, styrene butadienerubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as athickener were mixed in ion-exchanged water in a mass ratio ofC:SBR:CMC=98:1:1, thereby preparing negative electrode active materiallayer slurry. This negative electrode active material layer slurry wasapplied onto copper foil and then dried, thereby forming a negativeelectrode active material layer. The negative electrode active materiallayer was roll-pressed to have a predetermined density by rollingrollers, and then cut into a predetermined size, thereby obtaining anegative electrode sheet.

As a separator, a porous polyolefin sheet was prepared. The positiveelectrode sheets and the negative electrode sheets were alternatelystacked with separators interposed therebetween, thereby obtaining astacked-type electrode body.

Electrode terminals were attached to the stacked-type electrode body,and the resultant was inserted in a battery case constituted by analuminium laminated sheet. A nonaqueous electrolyte was injected intothe battery case. The nonaqueous electrolyte used was obtained bydissolving LiPF₆ as a supporting electrolyte at a concentration of 1mol/L in a mixed solvent including ethylene carbonate (EC) and ethylmethyl carbonate (EMC) in a volume ratio of EC:EMC=30:70 and adding 0.3mass % of vinylene carbonate to the resulting solvent mixture.Thereafter, the battery case was sealed, thereby obtaining an evaluationlithium ion secondary battery of Example 1.

Examples 2 to 6 and Comparative Examples 1 to 3

Evaluation lithium ion secondary batteries were produced by the samemethod as Example 1 except for using second lithium composite oxideparticles having an average primary particle size and the porosity shownin Table 1. It should be noted that the porosity of the second lithiumcomposite oxide particles was adjusted by changing synthesis conditions(specifically, pH of a reaction solution and a steering speed) of aprecursor hydroxide (i.e., Ni_(0.55)Co_(0.20)Mn_(0.25)(OH)₂) inproduction of the second lithium composite oxide particles.

<Measurement of Average Particle Sizes (D50) of First and Second LithiumComposite Oxide Particles>

Volume-based particle size distributions of first and second lithiumcomposite oxide particles were measured with a commercially availablelaser diffraction and scattering particle size distribution analyzer,and a particle size corresponding to a cumulative frequency of 50 vol %from the small-size particle side was determined as average particlesizes (D50) of first and second lithium composite oxide particles.

<Measurement of Average Primary Particle Size of Second LithiumComposite Oxide Particles>

A cross-sectional observation sample of second lithium composite oxideparticles was produced by a cross-section polisher process. An SEM imageof this sample was obtained by using an SEM. Longer diameters of 50 ormore randomly selected primary particles were obtained with imageanalysis type particle size distribution measurement software“Mac-View.” An average of the obtained longer diameters was calculatedand employed as an average primary particle size (d₂) of second lithiumcomposite oxide particles.

<Measurement of BET Specific Surface Areas of First and Second LithiumComposite Oxide Particles>

BET specific surface areas of the first and second lithium compositeoxide particles were measured by a nitrogen adsorption method with acommercially available surface area analyzer (“Macsorb Model-1208”manufactured by Mountech Co., Ltd.).

<Porosity Measurement of Second Lithium Composite Oxide Particles>

A cross-sectional observation sample of second lithium composite oxideparticles was prepared by cross-section polisher process, and an SEMimage thereof was obtained with a scanning electron microscope (SEM).From the SEM image, an area of the entire secondary particles and atotal area of all the voids in the secondary particles were obtained.From (total area of all voids/area of entire secondary particles)×100, aporosity (%) was calculated.

<Cycle Characteristic Evaluation>

As initial charging, each evaluation lithium ion secondary battery wascharged to 4.25 V with a constant current at a current density of 0.2mA/cm² under a temperature environment of 25° C., and then charged witha constant voltage of 4.25 V until the current density reaches 0.04mA/cm². Each evaluation lithium ion secondary battery was rested for 10minutes, and then discharged with a constant current to 3.0 V at acurrent density of 0.2 mA/cm².

Each evaluation lithium ion secondary battery was placed under atemperature environment of 25° C., charged with a constant current to4.18 V at a current density of 0.2 mA/cm², and then charged with aconstant voltage until the current density reaches 0.04 mA/cm².Thereafter, each evaluation lithium ion secondary battery was dischargedwith a constant current to 3.48 V at a current density of 0.2 mA/cm². Adischarge capacity at this time was obtained. This charging anddischarging process was defined as one cycle, and 400 cycles of thecharging and discharging process were repeatedly performed on eachevaluation lithium ion secondary battery. A discharge capacity at the400th cycle was obtained. From (discharge capacity at 400th cycle ofcharging and discharging process/discharge capacity at first cycle ofcharging and discharging process)×100, a capacity retention rate (%) wasobtained as an index of cycle characteristics. Table 1 shows results.

<Measurement of Volume Charge Capacity Density>

As initial charging, each evaluation lithium ion secondary battery wascharged to 4.25 V with a constant current at a current density of 0.2mA/cm² under a temperature environment of 25° C., and then charged witha constant voltage of 4.25 V until the current density reaches 0.04mA/cm². A charge capacity at this time was obtained.

A sample obtained by mixing first lithium composite oxide particles andsecond lithium composite oxide particles in a mass ratio of 5:5 andmolding the resulting mixture into a cylinder shape of 019 mm waspressed under a load of 70 kN for 30 seconds. A density of the pressedmixture was measured. The obtained density was multiplied by the chargecapacity described above, thereby calculating a volume charge capacitydensity (mAh/cm³). Table 1 shows results.

[Table 1]

TABLE 1 Second lithium composite oxide particles Average primaryCapacity Volume charge Porosity particle size retention capacity density(%) (μm) rate (%) (mAh/cm³) Example 1 1.1 1.8 93 704 Example 2 1.4 0.992 708 Example 3 2.0 0.6 93 706 Example 4 2.1 2.5 92 718 Example 5 2.40.6 92 706 Example 6 3.2 0.6 93 722 Comparative 0.3 2.6 89 714 Example 1Comparative 0.6 2.4 90 691 Example 2 Comparative 5.4 0.5 93 643 Example3

The results of Table 1 show that Comparative Examples 1 and 2 in whichthe second lithium composite oxide particles have significantly smallporosities show low capacity retention rates after charging anddischarging cycles, and thus, show low cycle characteristics. On theother hand, Comparative Example 3 in which the second lithium compositeoxide particles have a significantly large porosity shows a low volumecharge capacity density.

On the other hand, Examples 1 to 6 show both high cycle characteristicsand high volume charge capacity densities.

From the foregoing results, it can be understood that the positiveelectrode active material disclosed here can increase volume capacitydensity of a positive electrode of a nonaqueous electrolyte secondarybattery and can provide the nonaqueous electrolyte secondary batterywith high cycle characteristics.

Specific examples of the present disclosure have been described indetail hereinbefore, but are merely illustrative examples, and are notintended to limit the scope of claims. The techniques described inclaims include various modifications and changes of the aboveexemplified specific examples.

What is claimed is:
 1. A positive electrode active material comprising:monoparticulate first lithium composite oxide particles; and secondaryparticulate second lithium composite oxide particles, wherein an averageparticle size (D50) of the second lithium composite oxide particles islarger than an average particle size (D50) of the first lithiumcomposite oxide particles, and the second lithium composite oxideparticles have a porosity of 0.9% to 4.0%.
 2. The positive electrodeactive material according to claim 1, wherein the average particle size(D50) of the second lithium composite oxide particles is 12 μm to 20 μm.3. The positive electrode active material according to claim 1, whereinthe average particle size (D50) of the first lithium composite oxideparticles is 2 μm to 6 μm.
 4. The positive electrode active materialaccording to claim 1, wherein each of the first lithium composite oxideparticles and the second lithium composite oxide particles is particlesof a lithium composite oxide containing Ni and having a layeredstructure.
 5. The positive electrode active material according to claim4, wherein each of the first lithium composite oxide particles and thesecond lithium composite oxide particles is particles of a lithiumnickel cobalt manganese composite oxide.
 6. The positive electrodeactive material according to claim 5, wherein a content of nickel in allmetal elements except for lithium in the lithium nickel cobalt manganesecomposite oxide is 50 mol % or more.
 7. A nonaqueous electrolytesecondary battery comprising: a positive electrode; a negativeelectrode; and a nonaqueous electrolyte, wherein the positive electrodeincludes the positive electrode active material according to claim 1.